1. Field of the Invention
This invention relates to stepper motors, and in particular to permanent-magnet sheet-metal type stepper motors.
2. Description of Related Art
Stepper motors include a stator for producing a rotating multipole magnetic field and a multipole rotor for rotating about a central axis of the stator in synchronism with the rotating field. Typically the rotating magnetic field is effected by applying electrical pulses of continually changing phase to respective windings of the stator. Further details of typical stepper motor structure, operation and drive circuitry can be obtained from a publication entitled AIRPAX Stepper Motor Handbook published by Airpax Company, 604 West Johnson Avenue, P.O. Box 590, Cheshire, Conn., U.S.A.
Because such a motor is capable of converting electrical pulses into discrete mechanical rotational movements, it is particularly useful for controlling the rotary or linear position of an object coupled to its rotor. An example of one linear application which takes advantage of this capability is illustrated in FIG. 1. This Figure shows a stepper motor 10 which has been made by Airpax Company (part number A95228) for controlling the linear position of a shaped head 12 to control airflow 14 in a throttle body 16 of an internal combustion engine (not shown). The external surface of the head 12 is precisely shaped to conform to a neck surface 18 of the throttle body so that the linear position of the head regulates the magnitude of a clearance between the two surfaces.
Although it is not shown in FIG. 1, the head 12 is attached to a shaft which has a threaded end engaging an internal thread of the rotor in the stator contained in the motor housing 20. The shaft is prevented from rotating with the rotor by means of a separate part which has fingers projecting into respective longitudinal grooves in the shaft. The separate part is attached to an end of the stator, which itself includes a number of parts held together in two outer shell members which are fastened together. This arrangement functions well, but is complicated to assemble. Such complicated assembly of numbers of individual parts typifies the motor.